People would naturally be more concerned about themselves than others for survival purposes, as Darwin might have reasoned. Quite a lot of human psychology stems from the Survival-of-the-Fittest notion.

"I'm gonna get a tattoo to prove that I was in the Navy.""Oh? What are you going to get tattooed? An anchor or a life preserver? Oh, what about the battleship SS Missouri?""No, my mother has that.""Your mother has a battleship tattooed on her???""Yeah, whenever she wears a backless evening gown, it looks like it's sinking. I know what I'm going to get as a tattoo.""What?"My uniform."